1. Field of the Invention
The present invention is in the field of low pass filters for single tube color television cameras, color television cameras using solid state imagers and the like.
2. Description of the Prior Art
A single tube color television camera using a stripe color filter or a solid state color television camera using a solid state imager of a discrete picture element structure is usually designed to obtain an image output corresponding to each of the picture elements of an optical image by an optical spatial sampling of the image.
In a color television camera adapted to perform optical spatial sampling for the optical image, the fineness of the picture pattern that can be processed is determined by the sampling frequency. If the optical image contains a higher frequency component than that of the defined spatial frequency, spurious noises are generated resulting in colors having no connection with the optical image in the reproduced image. In view of the foregoing, an optical low pass filter has been incorporated in the optical imaging system for restricting the higher spatial frequency components in the optical image to prevent the generation of spurious noises in the conventional color television camera described above.
There has been proposed an optical low pass filter which is adapted to maintain a contrast as high as possible for spatial frequencies lower than the cut-off spatial frequency, utilizing the double refraction of a quartz device. However, this type of optical low pass filter suffers from a high material cost because of the quartz present, and requires a plurality of pieces of quartz in a color television camera using a solid state imager so that it is not suitable for mass production because of various problems in its manufacture.
There has also been suggested an optical low pass filter utilizing a phase grating based on the fact that the autocorrelation function of the pupil function of an optical system provides a transfer function for that system (the absolute value of which is referred to as MTF) in which an aberration is positively provided to the pupil function so as to obtain a desired optical characteristic.
An optical low pass filter utilizing a phase grating consisting of a thin film of a periodically repetitive stripe-like structure formed on the surface of a transparent substrate such as glass which is inserted into an optical system to provide a difference in optical distance between the transmission lights, that is, a phase difference due to the thin film of the stripe-like repetitive structure to thereby change the phase relationship of the pupil function and attain the characteristic of an optical low pass filter is shown, for example, in U.S. Pat. No. 3,821,795. In the conventional optical low pass filter utilizing a phase grating, a stripe-like thin film is deposited on the surface of a substrate in which the thin film has a repetitive structure of a rectangular or similar shape.
Reference is also invited to the article entitled "Optical Low-Pass Filter for a Single-Vidicon Color Television Camera" by Mino et al appearing in the April, 1972 issue of the Journal of the SMPTE, page 282. This article describes a rectangular wave phase grating prepared by means of vacuum evaporation of magnesium fluoride on a plane-parallel glass plate.
An article by Townsend appearing in the November 1972 issue of "Applied Optics", page 2463, describes a low pass filter of the rectangular wave phase grating type in which the grating is composed of dichromated gelatin.
In structures of the type described, the MTF value of the optical image in the lower spatial frequency region which is particularly important among the image constituent elements is low to reduce the contrast. Furthermore, the pattern of the repetitive structure may appear on the screen if the aperture spot of the imaging system is restricted. Since the difference of refractive index between the vapor-deposited substance forming the thin film of rectangular cross-section and air is large, the error caused by the thickness of the vapor-deposited substance has to be kept within several hundred Angstroms upon production, which provides a substantial difficulty in manufacturing.